Producing dental replacement parts like crowns and bridges requires the exact determination of the dental situation in the mouth of the patient. Otherwise, the dental replacement parts will not accurately fit.
For determining the dental situation in the mouth of a patient different methods are know. Besides imaging and computer based methods, a huge portion of this task is still accomplished by using conventional dental impression materials.
Dental impression materials can be classified according to their curing mechanism (e.g. addition curing or condensation curing). Dental impression materials can also be classified according to their consistency. Besides low viscous dental impression materials, there exists highly viscous, so-called putty like dental impression materials.
Dental impression materials are typically provided as two component systems which consist of a base and a catalyst paste and which are mixed before use. Different types of chemistry can be employed to formulate dental impression materials.
Often used are polyether impression materials, which cure by a cationic ring-opening polymerization of aziridines (e.g. Impregum™′ 3M ESPE), polysiloxanes which cure via a hydrosilation reaction (e.g. Imprint™3M ESPE), polysiloxanes which cure via a condensation mechanism (e.g. Xantropren™, Heraeus Kulzer) and mixtures of polyethers and siloxanes which cure via a hydrosilation mechanism (e.g. Senn™, GC).
The curing reaction of the aziridino moieties containing impression material is typically started by mixing a base paste containing the aziridino moieties bearing prepolymer and a catalyst paste containing a strong acid, especially a Lewis acid.
E.g., US 2004/0149164 relates to a mixture of elongated N-alkylaziridine prepolymers which can be used as a dental material. The mixture can contain various modifiers like finely divided fillers, pigments, thixotropic agents and surface-active substances.
U.S. Pat. No. 6,599,960 relates to storage-stable cationically polymerized preparations with improved hardening characteristics. The preparations can contain 0.0005 to 50 wt.-% of soluble and/or fine-particle organic and/or inorganic alkaline earth and/or alkali metal compounds. The preparation can be used for making dental impressions.
U.S. Pat. No. 3,926,636 relates to a light-curable composition consisting of a substance containing at least two aromatic or heteroaromatic o-nitrocarbinol ester groups of a certain structure and a compound having at least two aziridine groups or isocyanate groups. The light-curable composition is particularly suitable for the production of coatings and printing plates.
U.S. Pat. No. 4,167,618 (Schmitt) relates to a polymerization process for aziridine compounds. The polymerization process includes mixing an aziridine compound with an alkyl sulfonium salt.
WO 2011/133495 (3M) describes a radiation curable composition for taking a dental impression comprising a) a cationically hardenable compound comprising at least one aziridine moiety and b) a radiation sensitive starter, the radiation sensitive starter comprising an onium salt, a ferrocenium salt, a combination or mixture thereof.
The materials are typically pasty and cure in the mouth of the patient due to a chemical curing mechanism.
The materials are typically coloured. Colouring the materials is done to address a couple of needs.
From a patient's perspective a nicely coloured material is often more accepted than a non-coloured indifferent mass.
From a practitioner's perspective having a coloured material is desired to have a better contrast in the mouth of the patient, i.e. in order to be able to clearly determine where the material has already been applied and where not. This is even more important, when it has to be ensured that the cured material has been completely removed from the mouth of a patient after curing.
Once removed from the mouth of the patient, the obtained impression represents a negative image of the dental situation.
For producing a dental replacement part, the negative image has to be converted into a positive model or image.
This can be done either by filling the negative mould with plaster to obtain a positive model.
Alternatively, the surface of the negative image is scanned and the electronic data obtained further processed.
However, obtaining a suitable scan showing all the details of the surface of a rubber-elastic material is not easy.
In order to improve scannability of dental impression materials different approaches are meanwhile described.
WO 2008/064872 A2 (Kettenbach) also published as US 2010/0035210 A1 describes a method for producing a dental product, according to which a moulding, in the form of a negative mould, of a tooth or tooth stump is created using a moulding material, and this negative mould is scannned. In this respect, the moulding material may contain titanium oxide, zirconium oxide, zinc oxide and/or barium oxide.
WO 2006/108384 A1 (Dreve) relates to an optimized silicon material for digital optical data acquisition. The silicone material contains a metal oxide powder such as titanium oxide having a particle size below 50 μm.
WO 02/11678 A2 (S&C Polymer) also published as U.S. Pat. No. 7,625,442 describes a kit of parts consisting of a) a material for producing a shaped body and b) a metal powder, a powder of a metal alloy, a powder of a pigment having a metallic effect or a powder having a laminar structure.
DE 198 47 635 A1 (Wacker-Chemie) describes curable dental materials containing organopolysiloxane particles having a mean particle size between 5 and 200 nm. It is stated that the organopolysiloxane particles should be soluble in either of toluene, tetrahydrofuran or water.
All these approaches have in common that a certain amount of inorganic pigment or other light-reflective additive is added.
Adding a highly light-reflective additive, however, affects the brightness of the material. The initially desired colour is weakened and becomes less intense. This is not desired.
Besides the need for a dental material which is nicely coloured and easily scannable, the dental material needs to fulfill other requirements as well.
The above mentioned dental impression compositions based on so-called polyether chemistry curing via N-aziridino moieties are often the preferred choice for the practitioner, if a dental impression with a high accuracy is needed and desired.
However, sometimes removal of the cured composition based on those materials from the mouth of a patient is not easy. Some practitioners and patients rate the cured composition as too hard and/or not elastic enough.